But you can wrap that Map with Collections.unmodifiableMap() for an immutable version.
Indeed, but there is no way to reflect that in the return type, besides the JavaDoc of course. If you try and modify it later, you'll get a runtime exception, which is a pretty crappy thing. One of the things I dislike most about the Collections API is that Mutable interfaces are not extensions of Immutable interfaces with just the addition of modifiers. I should be forced to say that I'm returning a Mutable version of a data structure in my return type.
> is that Mutable interfaces are not extensions of Immutable interfaces with just the addition of modifiers. I should be forced to say that I'm returning a Mutable version of a data structure in my return type.
It wouldnt work. If MutableList<E> extends ImmutableList<E> then every MutableList is also an ImmutableList. This way you can be forced to return a MutableList, but not an ImmutableList.
Fair point, you're absolutely correct. Admittedly I haven't given this a TON of thought, because it seems like something that can't be added anymore. Maybe the interfaces should be siblings, but since Java doesn't have a disjoint type, you might run into problems if something says it implements both MutableList and ImmutableList. Not sure if Java's type system is strong enough to deal with this problem.
I think the Map interface should have been composed of two interfaces, ReadableMap and WriteableMap. Then APIs could return ReadableMap, which would lack the mutator methods altogether.
Then a Map would be a ReadableMap, but a ReadableMap wouldn't be a Map. If a method expects a Map, you would't be able to put a ReadableMap (or a WriteableMap) into it.
My solution so far is to avoid to return a map or to expect a map as argument.
Not quite. If the original collection is modified, the "immutable" view will reflect that change.
Plus, you can't statically know if the collection you're passed is immutable or not. Trying to modify it will just blow up at runtime.
If I have:
private final Map<String, String> map = new HashMap<>();
I cannot say map = someOtherMap;, but I can still say map.put("key", "value");.